1. Field of the Invention
This invention relates to laser diodes, and more particularly to nitride based semiconductor laser diodes and methods for fabricating same.
2. Description of the Related Art
A laser is a device that produces a beam of coherent light as a result of stimulated emission. Light beams produced by lasers can have high energy because of their single wavelength, frequency, and coherence. A number of materials are capable of producing a lasing effect and include certain high-purity crystals (such as ruby), semiconductors, certain types of glass, certain gasses including carbon dioxide, helium, argon and neon, and certain plasmas.
More recently there has been increased interest in lasers made of semiconductor materials. These devices typically have a smaller size, lower cost, and have other related advantages typically associated with semiconductor devices. Semiconductor lasers are similar to other lasers in that the emitted radiation has spacial and temporal coherence, and like other lasers, semiconductor lasers produce a beam of light that is highly monochromatic (i.e. of narrow bandwidth) and is highly directional. Overall, semiconductor lasers provide very efficient systems that are easily modulated by modulating the current directed across the devices. Additionally, because semiconductor lasers have very short photon lifetimes, they can be used to produce high-frequency modulation.
One type of semiconductor laser diode is referred to as an edge emitting laser where the stimulated emission is from the side surface or edge of the laser diode. These devices typically have epitaxial layers in the form of waveguiding or reflective elements (cladding layers) with a light generating active region between the reflective elements. Additional layers can be included between the reflective elements to form a laser cavity. The edges of the laser diode can be cleaved during manufacturing to form edge reflective surfaces. A total reflectivity (TR) material can cover one edge, and an anti reflectivity (AR) material can cover the opposite edge. Light from the active region is reflected between the edges and within the cavity by the reflective elements, with stimulated emission emitting from the edge with the AR material.
A known characteristic of laser diodes (and light emitting diodes) is that the frequency of radiation that can be produced by the particular laser diode is related to the bandgap of the particular semiconductor material. Smaller bandgaps produce lower energy, shorter wavelength photons, while wider bandgaps produce higher energy, shorter wavelength photons. One semiconductor material commonly used for lasers is indium gallium aluminum phosphide (InGaAlP), which has a bandgap that is generally dependant upon the mole of atomic fraction of each element present. This material, regardless of the different element atomic fraction, produces only light in the red portion of the visible spectrum, i.e., about 600 to 700 nanometers (nm).
Laser diodes that produce shorter wavelengths not only produce different colors of radiation, but offer other advantages. For example, laser diodes, and in particular edge emitting laser diodes, can be used with optical storage and memory devices (e.g. compact disks (CD) digital video disks (DVD), high definition (HD) DVDs, and Blue Ray DVDs). Their shorter wavelength enables the storage and memory devices to hold proportionally more information. For example, an optical storage device storing information using blue light can hold approximately 32 times the amount of information as one using red light, using the same storage space. There are also applications for shorter wavelength laser in medical systems and projection displays. This has generated interest in Group-III nitride material for use in laser diodes, and in particular gallium nitride (GaN). GaN can produce light in the blue and ultra violet (UV) frequency spectrums because of its relatively high bandgap (3.36 eV at room temperature). This interest has resulted in developments related to the structure and fabrication of Group-III nitride based laser diodes [For example see U.S. Pat. Nos. 5,592,501 and 5,833,706 to Edmond et al].
Group-III nitride laser diodes can require relatively high threshold currents and voltages to reach laser radiation because of optical and electrical inefficiencies. These elevated current and voltage levels can result in heat being generated during laser diode operation. In certain applications, laser diodes are driven by a pulsed signal that results in pulsed laser light being emitted from the laser diode. The heat generated within the laser diode typically does not present a problem during pulsed laser diode operation because the laser diode has the opportunity to cool during the lows of the signal. For other important applications, however, it can be desirable to drive the laser diode with a continuous wave (CW). CW operation is particularly applicable to operation with optical storage devices that can require a continuous light source for data storage and retrieval. Driving many current Group-III based laser diodes with a CW having the threshold current and voltage necessary for laser emission can result in heating that can damage or destroy the laser diode. Heat sinks or other cooling methods/devices can be employed to reduce operating heat within these laser diodes, but the methods/devices can increase the cost and complexity of the devices and can require additional space.